Julia Goerges upset top-ranked Caroline Wozniacki 7-6 (7-3), 6-3 in the final of the Porsche Grand Prix on Sunday.
The 22-year-old German won in Bad Gastein, Austria, last year but had never captured a title in one of the WTA's premier events.
"I only know I won, but I don't know how," Goerges said. "Playing against Caroline is like climbing a high hill."
Goerges becomes the first German to win the tournament since Anke Huber in 1996 and will now move to a career-best ranking of No. 27.
Wozniacki was looking for her fourth title of the year.
"Julia played very well and very aggressive. I wasn't good enough today," said Wozniacki, who had not dropped a set in the tournament until the final and had not lost a match on clay this year.
Goerges collected a sports car donated by the title sponsor of the indoor tournament.
"I walked past this car every day to practice. I never imagined that I would be driving it," Goerges said.